qt-everywhere-src-5.14.1/qtbase/examples/widgets/doc/src/analogclock.qdoc

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/*!
    \example widgets/analogclock
    \title Analog Clock Example
    \ingroup examples-widgets
    \brief The Analog Clock example shows how to draw the contents of a
    custom widget.

    \borderedimage analogclock-example.png
    \caption Screenshot of the Analog Clock example

    This example also demonstrates how the transformation and scaling
    features of QPainter can be used to make drawing custom widgets
    easier.

    \section1 AnalogClock Class Definition

    The \c AnalogClock class provides a clock widget with hour and minute
    hands that is automatically updated every few seconds.
    We subclass \l QWidget and reimplement the standard
    \l{QWidget::paintEvent()}{paintEvent()} function to draw the clock face:

    \snippet widgets/analogclock/analogclock.h 0

    \section1 AnalogClock Class Implementation

    \snippet widgets/analogclock/analogclock.cpp 1

    When the widget is constructed, we set up a one-second timer to
    keep track of the current time, and we connect it to the standard
    \l{QWidget::update()}{update()} slot so that the clock face is
    updated when the timer emits the \l{QTimer::timeout()}{timeout()}
    signal.

    Finally, we resize the widget so that it is displayed at a
    reasonable size.

    \snippet widgets/analogclock/analogclock.cpp 8
    \snippet widgets/analogclock/analogclock.cpp 10

    The \c paintEvent() function is called whenever the widget's
    contents need to be updated. This happens when the widget is
    first shown, and when it is covered then exposed, but it is also
    executed when the  widget's \l{QWidget::update()}{update()} slot
    is called. Since we connected the timer's
    \l{QTimer::timeout()}{timeout()} signal to this slot, it will be
    called at least once every five seconds.

    Before we set up the painter and draw the clock, we first define
    two lists of \l {QPoint}s and two \l{QColor}s that will be used
    for the hour and minute hands. The minute hand's color has an
    alpha component of 191, meaning that it's 75% opaque.

    We also determine the length of the widget's shortest side so that we
    can fit the clock face inside the widget. It is also useful to determine
    the current time before we start drawing.

    \snippet widgets/analogclock/analogclock.cpp 11
    \snippet widgets/analogclock/analogclock.cpp 12
    \snippet widgets/analogclock/analogclock.cpp 13
    \snippet widgets/analogclock/analogclock.cpp 14

    The contents of custom widgets are drawn with a QPainter.
    Painters can be used to draw on any QPaintDevice, but they are
    usually used with widgets, so we pass the widget instance to the
    painter's constructor.

    We call QPainter::setRenderHint() with QPainter::Antialiasing to
    turn on antialiasing. This makes drawing of diagonal lines much
    smoother.

    The translation moves the origin to the center of the widget, and
    the scale operation ensures that the following drawing operations
    are scaled to fit within the widget. We use a scale factor that
    let's us use x and y coordinates between -100 and 100, and that
    ensures that these lie within the length of the widget's shortest
    side.

    To make our code simpler, we will draw a fixed size clock face that will
    be positioned and scaled so that it lies in the center of the widget.

    The painter takes care of all the transformations made during the
    paint event, and ensures that everything is drawn correctly. Letting
    the painter handle transformations is often easier than performing
    manual calculations just to draw the contents of a custom widget.

    \image analogclock-viewport.png

    We draw the hour hand first, using a formula that rotates the coordinate
    system counterclockwise by a number of degrees determined by the current
    hour and minute. This means that the hand will be shown rotated clockwise
    by the required amount.

    \snippet widgets/analogclock/analogclock.cpp 15
    \snippet widgets/analogclock/analogclock.cpp 16

    We set the pen to be Qt::NoPen because we don't want any outline,
    and we use a solid brush with the color appropriate for
    displaying hours. Brushes are used when filling in polygons and
    other geometric shapes.

    \snippet widgets/analogclock/analogclock.cpp 17
    \snippet widgets/analogclock/analogclock.cpp 19

    We save and restore the transformation matrix before and after the
    rotation because we want to place the minute hand without having to
    take into account any previous rotations.

    \snippet widgets/analogclock/analogclock.cpp 20
    \codeline
    \snippet widgets/analogclock/analogclock.cpp 21

    We draw markers around the edge of the clock for each hour. We
    draw each marker then rotate the coordinate system so that the
    painter is ready for the next one.

    \snippet widgets/analogclock/analogclock.cpp 22
    \snippet widgets/analogclock/analogclock.cpp 23

    The minute hand is rotated in a similar way to the hour hand.

    \snippet widgets/analogclock/analogclock.cpp 25
    \codeline
    \snippet widgets/analogclock/analogclock.cpp 26

    Again, we draw markers around the edge of the clock, but this
    time to indicate minutes. We skip multiples of 5 to avoid drawing
    minute markers on top of hour markers.
*/